The invention relates to gas turbine shrouds, and, more particularly, to a gas turbine shroud made from ceramic material.
Industrial gas turbines typically have the first stage of the turbine section designed without flow guiding shrouds attached to the turbine airfoils, the static turbine shrouds perform the important function of maintaining the hot combustion gases in the proper annular area thus preserving the flow path pressure while the turbine airfoil can properly extract energy. The shroud performs this function by tightly controlling the clearance between the internal surface of the shroud and the tip of the turbine airfoil.
A conventional metallic turbine shroud has temperature limits that require gas turbine operating airflow to keep it cooled below its critical temperature limit. The source of this cooling air is from the compressor discharge that has not yet had energy from the combustor added to it. Diverting air for cooling the metallic turbine inner shroud, however, reduces gas turbine efficiency.
In addition a relatively large number of metallic inner shrouds are needed so that a relatively large number of expansion gaps between the inner shrouds, are available to negate the thermal expansion effects of the metallic inner shrouds.
Replacing the metallic inner shrouds with higher temperature ceramic inner shrouds substantially reduces or even eliminates the need for the cooling airflow. Eliminating the need for cooling air to reduce shroud temperature allows more air to enter the combustor and have fuel energy added to it so that the turbine section can extract it. Because of this extra amount of combusted air, the gas turbine can extract more energy to drive the generator thereby creating more electrical energy output and at a higher gas turbine efficiency (simple and combined cycle). Thus, replacement of the current metallic inner shrouds with higher temperature capable composite ceramic shrouds serves to eliminate or reduce performance robbing cooling air.
Using ceramic shrouds having a higher temperature capability may also allow a fewer number of inner shrouds to be used than the current metallic design. The savings realized by the invention can include fewer number of parts, fewer leak paths, and higher firing temperature capability.